Tbi 350 bogs and stalls at half throttle

[Matts88]

I have a 1988 Chevy suburban with the 350 tbi in it. I had a problem where it idled fine, revved up good in park/neutral, but when you put it into gear and give it more then the slightest bit of gas is would bog down and stall out. I have since replaced the vacuum lines and checked the Egr. Egr is good and after replacing the lines I have gained roughly half my throttle back, but I've come to a stand still, I've replaced almost all sensors, tps, map, o2, Egr solenoid, and haven't had any changes. The cel is on for code 33 which google said was low vacuum to the map sensor. If anybody has any ideas/suggestions they would be greatly appreciated.

 

[vr1967]

Is the electrical side of your map sensor good?

To test:
There are 3 wires going to the sensor. 5v hot, a feed back, and a ground. The feed back is the middle wire.
With a multi meter, check that you have 5v going to the sensor. Plug it in, and tap into your feed back wire. I used a straightened paperclip.
Turn the key on, and you should have right at 5v on the center wire. Connect a vacuum pump to the port, and for every 5in of vacuum, you should lose app 1v (with no vacuum, should be 5v, with 5” of vacuum, should read 4v, 10” of vacuum, should read 3v, etc)

 

[Matts88]

it reads 4.98 volts on the multimeter set on dc, with the negative in the middle and the positive on the hot terminal. I'm not sure what you meant with the paper clip though?

 

[Erik the Awful]

Your symptoms are indicative of a weak fuel pump. That it came back a bit with your fixes tells me that you likely have multiple problems.

First and most important, what's your fuel pressure? What's your fuel pressure when you press the gas in gear and it tries to stall?

 

[Scooterwrench]

What vr is trying to explain is back probe the center terminal with the connector connected to the sensor,ignition on,engine off. Use the piece of paper clip to slide in the wire seal and make a connection with the center terminal. You touch one lead of the multimeter to the paper clip and the other lead to a good ground. As you apply vacuum to the MAP sensor port the voltage should go down as he describes.

 

[Drunkcanuk]

I had the same symptoms on my 91 5.7. Shot a few parts at it. Then finally dropped the tank to do the fuel pump. The rubber hose was full of holes, the ethanol in the gas ate it up. The new pump came with a hose designed for ethanol gas.
It would rev fine in neutral and park, but as soon as it had a load, stumbled, sputtered and fell on its face.
Can you hear fuel spraying around in the tank at 1/4 tank? A few people have reported that as a symptom as well.

 

[Scooterwrench]

I had the same symptoms on my 91 5.7. Shot a few parts at it. Then finally dropped the tank to do the fuel pump. The rubber hose was full of holes, the ethanol in the gas ate it up. The new pump came with a hose designed for ethanol gas.
It would rev fine in neutral and park, but as soon as it had a load, stumbled, sputtered and fell on its face.
Can you hear fuel spraying around in the tank at 1/4 tank? A few people have reported that as a symptom as well.

Yup,good suggestion. Fuel pressure test will confirm without pulling the tank or lifting the bed.

 

[Matts88]

I couldn't find the adapters for the fuel lines anywhere in town, neither the in line one or the one behind the throttle body. I attempted to shove the rubber hose over the nozzles but it didn't hold enough to read the pressure. I found some that look like they'll work on Amazon so I'll order them and update when I get a reading. As for the sensor I put the paper clip in the center wire, put the red on it and the black on a good ground. It's reading 5 volts with ignition on, engine off, but I haven't found a vacuum pump yet, but I'm currently heading into town to look for one

 

[Matts88]

No vacuum 4.77v, 5psi 4.98, 10 psi 4.98 again.so that means the wiring is bad?

 

[Schurkey]

No vacuum 4.77v, 5psi 4.98, 10 psi 4.98 again.so that means the wiring is bad?

WHICH wire are you checking, and how do you have the meter leads positioned?

This is why using a scan tool is easier, faster, and has less chance of error.